The institute of electrical and electronics engineers (IEEE) 802.16 standard provides a technique and protocol for supporting broadband wireless access. The standardization had been conducted since 1999 until the IEEE 802.16-2001 was approved in 2001. The IEEE 802.16-2001 is based on a physical layer of a single carrier (SC) called ‘WirelessMAN-SC’. The IEEE 802.16a standard was approved in 2003. In the IEEE 802.16a standard, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ are further added to the physical layer in addition to the ‘WirelessMAN-SC’. After completion of the IEEE 802.16a standard, the revised IEEE 802.16-2004 standard was approved in 2004. To correct bugs and errors of the IEEE 802.16-2004 standard, the IEEE 802.16-2004/Cor1 was completed in 2005 in a format of ‘corrigendum’.
A multiple input multiple output (MIMO) technique improves efficiency of data transmission and reception by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas. Ever since the MIMO technique was introduced from the IEEE 802.16a standard, the MIMO technique has been constantly updated up to now.
The MIMO technique can be classified into a spatial multiplexing technique and a spatial diversity technique. According to the spatial multiplexing technique, different streams are simultaneously transmitted and thus data can be transmitted at a high speed without increasing a system bandwidth. According to the spatial diversity technique, same streams are transmitted through multiple Tx antennas to obtain diversity, thereby increasing data reliability.
Hereinafter, a downlink denotes a communication link from a base station (BS) to a mobile station (MS), and an uplink denotes a communication link from the MS to the BS.
In general, the BS schedules uplink and downlink radio resources in a wireless communication system. The radio resources are used for data transmission in the wireless communication system. For example, timeslots are resources in a time division multiple access (TDMA) system. Codes and timeslots are resources in a code division multiple access (CDMA) system. Subcarriers and timeslots are resources in an orthogonal frequency division multiple access (OFDMA) system. The respective resources can be defined to be orthogonal in a time, code, or frequency domain, so that interference does not occur to other users in the same cell or sector.
The MS can transmit a feedback message through uplink. The feedback message includes channel information required for data transmission. By using the feedback message received from the MS, the BS can schedule a radio resource and can transmit data. A closed-loop mode is a mode in which data is transmitted by compensating for channel information included in the feedback message received from the MS. An open-loop mode is a mode in which data is transmitted without compensating for the channel information included in the feedback message received from the MS. The feedback message may not be transmitted in the open-loop mode. Even if the feedback message is transmitted, the BS may not use the channel information included in the feedback message. Generally, in the communication system, the open-loop mode can be used in a channel environment where the MS moves with a high speed, and the closed-loop mode can be used in a channel environment where the MS moves with a slow speed. More specifically, the open-loop mode is used when the MS moves with a high speed since a channel variation is significant and thus channel information included in feedback message is unreliable. In addition, the closed-loop mode is used when the MS moves with a slow speed since a channel variation is relatively less significant and thus the channel information included in the feedback message is reliable and is less sensitive to delay.
Examples of the feedback message transmitted by the MS may include a scheduling request that requests radio resource allocation such as a bandwidth request (BR), an acknowledgment (ACK)/negative-acknowledgment (NACK) signal that is a response for downlink data transmission, a channel quality indicator (CQI) that indicates downlink channel quality or MIMO information, etc.
However, when the MS generates and transmits independent feedback messages for the respective types of information (e.g., CQI, MIMO, etc.), many overheads are caused by limited radio resources. Therefore, there is a need for a method of effectively transmitting a feedback message by combining different types of channel information.